Second conditional when making plans for the future: "If it rained tomorrow, I would go to the cinema."

• If it rained tomorrow, I would go to the cinema.
• If it didn't rain tomorrow, I would go to school.

Which conditional is the best for this use case? Is it correct to use the Second Conditional or do I have to use the First? ("If it rains tomorrow, I will go to the cinema.")

You could say

If it doesn't rain tomorrow, I will go to school.

or, more pompously,

If it weren't to rain tomorrow, I would go to school.

The problem is, you're mixing past and future conditions. In both cases, it can't have rained tomorrow.

It depends on how likely the conditional event is, or how likely you would like to portray it as.

For a conditional clause expressing a possible future event, you have two choices: (i) put the inflected verb in present form, or (ii) put the inflected verb in preterite form. Use option (ii) if the contemplated event is unlikely or being posed as if contrary to fact. Use of the preterite does not in such a case imply past tense (and it obviously couldn't, since we're talking about a future event), it is instead called the "modal preterite", since it is used for a non-indicative mood.

The second clause should have its inflected verb in the same form as the conditional clause does.

If it rains tomorrow, I will go to the cinema. (It is likely to rain tomorrow)
If it rained tomorrow, I would go to the cinema. (There is no reason to believe that it will rain tomorrow, but it just might happen)